This invention relates to improvements in a velocity control apparatus for an elevator.
FIG. 1 is a circuit diagram which shows an example of a velocity control apparatus for an elevator employing an inverter. Referring to the figure, numeral 1 designates a power source, numeral 2 a main circuit switch (a coil is not shown, and only a contact is shown), numeral 3 a converter which converts alternating current into direct current, numeral 4 a smoothing capacitor which smooths the output voltage of the converter 3, and numeral 5 the inverter which inverts direct current into alternating current. Numerals 6 and 7 designate current detectors which detect the output currents of the inverter 5. Numeral 8 indicates a hoist motor, numeral 9 a velocity detector which is mounted on the hoist motor 8, numeral 10 a sheave winder, numeral 11 a traction rope which connects a cage 12 and a balance weight 13, and numeral 14 a velocity pattern generator. Numeral 15 denotes a velocity calculating circuit which compares the output of the velocity pattern generator 14 with the output of the velocity detector 9 and then provides current commands to current control circuits 16 - 18. These current control circuits 16, 17 and 18 correspond to the U-phase, V-phase and W-phase, respectively.
FIG. 2 shows the details of any of the current control circuits 16, 17 and 18 shown in FIG. 1, all of which have the same arrangements. Referring to the figure, numeral 19 indicates an arithmetic circuit which compares the output 15a of the velocity calculating circuit 15 and the output 6a of the current detector 6 and them amplifies the compared result. This arithmetic circuit 19 is composed of a plurality of resistors and an operational amplifier 19a. An output from the operational amplifier 19a of the arithmetic circuit 19 is pulse-width-modulated by the comparator 20a of a comparison circuit 20, to provide a PWM output, which drives the transistors of the inverter 5 shown in FIG. 1 through a base drive circuit 21.
In such an arrangement, current detectors employing Hall devices are usually used as the current detectors 6 and 7. The current detectors 6 and 7, however, have had the problem that their offset voltages fluctuate due to temperature fluctuations or secular changes. More specifically, even when the bases of the transistors of the inverter 5 are kept cut off, the current detector 6 or 7 provides an output, due to the offset voltage. This offset voltage is not always constant, and it fluctuates. Therefore, the operations of the current control circuits 16, 17 and 18 which operate by receiving the outputs of the current detectors 6 and 7 vary according to the fluctuations of the offset voltages. As a result, D.C. components appear in the output currents of the inverter 5, and the hoist motor 8 generates a torque ripple attributed to the D.C. components. The torque ripple is transmitted to the cage 12 through the sheave 10 as well as the traction rope 11, to give an unpleasant feeling to passengers in the cage 12 disadvantageously.